The term “masterbatch” refers to a composition obtained by a process of first mixing (e.g., melt mixing) an additive with a small amount of a carrier resin, such as a polyolefin resin, resulting in a “masterbatch.” The masterbatch is then mixed with a remaining bulk of the carrier resin, or with a bulk of another resin. In certain situations, the use of a masterbatch may result in better mixing of the additive in the final mixture than were the additive to be mixed directly in the bulk of the resin.
When a masterbatch is used to improve product performance or appearance of a polymer, particularly a polyolefin resin, it may be desirable that the carrier resin used in the masterbatch also be a polyolefin resin. However, often, the concentration of additives that can be effectively dispersed in a polyolefin carrier resin is low. For instance, a typical nucleator can only be loaded up to 4 weight percent in high density polyethylene (HDPE) and up to 10 weight percent in low density polyethylene (LDPE). These additive loading levels may be too low to have practical use in an industrial setting. To reach a higher concentration of additive in masterbatches, rubber and dispersants have traditionally been used to mix with additives in heated roll mills or kneaders. Such a process is labor intensive and more costly than the extrusion and pelletizing technique. Moreover, the resulting masterbatch often imparts undesirable substances, such as rubber and dispersants, to the polyolefin matrix.